Metals supported on porous media are important for use as sensors, separators or supported catalysts. Such supported catalysts can be used for chemical transformations such as hydrogenations. These supported catalysts can be used in continuous flow systems or batch reactions and permit the recovery and recycling of the catalysts, which are often very expensive and frequently difficult to remove when not associated with the supporting media. Common supporting porous media are carbon, silica, alumina, and organic polymers. Often organic polymers are incompatible with the solvents or temperatures used under the reaction conditions and there use is generally more limited in scope relative to that of carbon and inorganic oxides.
Fabrication of porous supporting media loaded with metal or other particles is difficult. The porous medium is frequently grown around the particles. Examples of the growth of the medium around the particles are the hydrolysis and condensation of a tetralkyloxysilane about dispersed particles to yield a porous silica containing dissimilar particles and the dispersion of particles in an organic polymer solution followed by removal of the solvent and pyrolysis of the polymer in an inert atmosphere to form a carbon foam with dispersed particulates. Although the size of the particle is readily controlled in this manner, these particles can be encapsulated by the support such that many or all particles are not available at the surface of the pores where they are needed for their effective use as a catalyst.
An alternative approach to the formation of a porous media around preformed particles is the formation of particles from a fluid precursor infused into a porous media. For example, the porous media can be put in contact with a soluble metal salt solution which can then be transformed into metal particles, often by a series of oxidation and reduction reactions to precipitate a metal oxide in the pores of the media followed by reduction of the oxides to metal particles.
The infusion of the metal into a porous media can be carried out by vapor deposition of a metal onto and into the media. Under well controlled conditions discreet metal islands can be formed on the media with a continuous metal film resulting upon further deposition, limiting the surface area of the metal to no more than the surface area of the porous medium and hence defining the maximum effectiveness of the structure as a catalyst. As the metal is deposited in intimate contact with the surface of the porous medium a large proportion of the metal surface is not available as a catalyst.
The infusion of preformed particles into porous media has been problematic. The infusion of single particles dispersed in a liquid medium requires that the particle is stabilized as agglomeration of the small particles or the coalescence of the small particles into a large particle can inhibits the filing of the pores of the media. Additionally coalescence or aggregation can reduce the effective surface area of the particles and hence their performance for a desired application. Additives and other surfactants and other stabilizers can be used to inhibit the aggregation of the particles, however, the removal of the additive is often difficult yet frequently required to provide a highly desired active surface for catalysis and other applications.
Hence a method that readily generates a porous medium with nanoparticles decorating the surface of the pores in a manner where virtually all particles can contribute to the activity of the catalyst, sensor, or other application remains a need.